From Wikipedia:

DeskMate was a software application that provided an operating environment that competed with early versions of Microsoft Windows. It originally was made for Tandy's TRS-DOS Operating System and for their TRS-80 line of computers, but eventually shifted to PC, where it was developed using C and Assembly. Like Windows (and other competitors from the time, such as GEM), it was not a full operating system, requiring a separate disk operating system in order to function. Although the initial PC ports would only run on Tandy's PCs (such as the Tandy 1000), the introduction of the true PC-compatible computers such as the Tandy 3000 resulted in the software later being made available for other compatibles.